This is a question reporters sometimes ask musicians: "So, tell me - who is your fanbase?" It is not an original question. I probably put it to at least 40 different artists over the course of my career until I realised I was always getting the exact same answer: "Oh, you know. All sorts of people." And this, I suppose, is generally an honest response.

If you scrutinise the followers of any significant rock group, you will predominantly find unrelated, nondescript quasi-hipsters who represent nothing in particular; the clearest sign that any entity has become mainstream popular is when it appeals to random people who don't really care. Asking a band about who inhabits their fanbase tells you nothing. A better question is this: "Who are your fans that care too much?" Because it's always the fringe lunatics who matter more.

As I stare at the various images in these photographs (see gallery above), it occurs to me that I have never tried to look like a musician (at least not in public). I wore a lime cardigan sweater after Nirvana appeared on MTV Unplugged, but that was mostly due to the weather; I used to wear preposterously ripped jeans like Joe Elliott on Def Leppard's Hysteria tour, but that's only because I was stupid and broke. My fashion choices were conscious, but they were also shallow and temporary. I was weird, but I wasn't fanatical. My perspective was limited by my logic.

What is far more fascinating is the way true fanatics can see aesthetic consistencies within the seemingly unremarkable. Until glancing at these photographs, I had never realised that the defining quality of a diehard Radiohead fan was the wearing of a diagonal strap across one's torso. What does that even mean? What do these people hear on Bodysnatchers that makes this visual determination so self-evident? It's not like this extraneous strap is some kind of widespread youth trend that lead guitarist Jonny Greenwood is spearheading; this is not like thousands of teenagers getting Beatles haircuts because that's the style of the time. It is not a look that will spill over into conventional society.

This personal choice has something to do with proving that you are not like a normal person, and it suggests you understand something about Radiohead that the average citizen cannot see or hear or feel. It goes to the nucleus of why pop music matters - at its highest levels of achievement, rock'n'roll is supposed to serve as "lifestyle music". It should have the potential to inform every single decision about being alive. And for these photogenic goofballs, it obviously does.

As I've said, I am not an authentic fanatic; I lack grit. But I spend a lot of time thinking about fanatics and I've concluded that the following 10 artists have the most dedicated, least rational fan followings:

The obvious follow-up question, of course, is why. Why do these particular musicians engender such sincere, unadulterated idolatry? In the case of Sublime, the answer is the evergreen popularity of recreational drug use; in the case of Springsteen, working-class optimism. But the other examples are more complex. My suspicion is that highly prolific artists - especially ones slagged off by cultural elitists - unknowingly present themselves in such a way that audiences recognise elements of themselves within the sonic iconography. If a musician a) ignores public criticism while b) producing a substantial body of work, an unwavering fanbase will self-select itself. They will see the artist as extra-human, will feel they "understand" how that artist's music reflects (and replicates) their own experience, and will immerse themselves in the musician's catalogue. The Smiths did not exist to be liked by anyone; either you loved them or you didn't care. Within the idiom of fanaticism, there is no benefit to emotional caution. You have to go all the way.

Which brings us back to the pictures in the gallery above: they are group portraits of those who have gone further than all the way. These people do not simply allow an artist to represent who they are; they are now trying to represent that artist to other people. Spanish philosopher George Santayana described this process as "redoubling your effort when you have forgotten your aim". He was not incorrect. It is perfectly normal to love the music of Rod Stewart, and not unnatural to feel that appreciating Never A Dull Moment says something essential about who you are; it is, however, pretty goddamn weird to try physically to resemble Rod Stewart while watching him perform in concert. It's hard to imagine what the best-case scenario would be for any man pursuing this goal; a hypothetical situation in which Stewart sees him from the stage and decides the two of them should become friends, despite the fact that Rod Stewart is not known for hanging out with people who look exactly like him? All things considered, it would probably make more sense to show up at a Rod Stewart concert looking exactly like Ron Wood.

Yet within the competitive context of life, these fanatics are almost certainly the winners. The purpose of good art is always twofold: it's supposed to help us see ourselves, and it's supposed to help us understand something greater. These people can do that simply by looking in a mirror. I mean, who needs music when you already have yourself?

· Photographs taken from The Disciples, by James Mollison, to be published by Chris Boot in October at £40. Prints will be exhibited at New York's Hasted Hunt Gallery, June 12-August 16